/*
 * Copyright (C) 2012 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.base;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.CharMatcher.NamedFastMatcher;

import java.util.BitSet;

/**
 * An immutable version of CharMatcher for smallish sets of characters that uses a hash table with
 * linear probing to check for matches.
 *
 * @author Christopher Swenson
 */
@GwtIncompatible // no precomputation is done in GWT
@ElementTypesAreNonnullByDefault
final class SmallCharMatcher extends NamedFastMatcher
{
    static final int MAX_SIZE = 1023;
    private final char[] table;
    private final boolean containsZero;
    private final long filter;

    private SmallCharMatcher(char[] table, long filter, boolean containsZero, String description)
    {
        super(description);
        this.table = table;
        this.filter = filter;
        this.containsZero = containsZero;
    }

    private static final int C1 = 0xcc9e2d51;
    private static final int C2 = 0x1b873593;

    /*
     * This method was rewritten in Java from an intermediate step of the Murmur hash function in
     * http://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp, which contained the
     * following header:
     *
     * MurmurHash3 was written by Austin Appleby, and is placed in the public domain. The author
     * hereby disclaims copyright to this source code.
     */
    static int smear(int hashCode)
    {
        return C2 * Integer.rotateLeft(hashCode * C1, 15);
    }

    private boolean checkFilter(int c)
    {
        return 1 == (1 & (filter >> c));
    }

    // This is all essentially copied from ImmutableSet, but we have to duplicate because
    // of dependencies.

    // Represents how tightly we can pack things, as a maximum.
    private static final double DESIRED_LOAD_FACTOR = 0.5;

    /**
     * Returns an array size suitable for the backing array of a hash table that uses open addressing
     * with linear probing in its implementation. The returned size is the smallest power of two that
     * can hold setSize elements with the desired load factor.
     */
    @VisibleForTesting
    static int chooseTableSize(int setSize)
    {
        if (setSize == 1)
        {
            return 2;
        }
        // Correct the size for open addressing to match desired load factor.
        // Round up to the next highest power of 2.
        int tableSize = Integer.highestOneBit(setSize - 1) << 1;
        while (tableSize * DESIRED_LOAD_FACTOR < setSize)
        {
            tableSize <<= 1;
        }
        return tableSize;
    }

    static CharMatcher from(BitSet chars, String description)
    {
        // Compute the filter.
        long filter = 0;
        int size = chars.cardinality();
        boolean containsZero = chars.get(0);
        // Compute the hash table.
        char[] table = new char[chooseTableSize(size)];
        int mask = table.length - 1;
        for (int c = chars.nextSetBit(0); c != -1; c = chars.nextSetBit(c + 1))
        {
            // Compute the filter at the same time.
            filter |= 1L << c;
            int index = smear(c) & mask;
            while (true)
            {
                // Check for empty.
                if (table[index] == 0)
                {
                    table[index] = (char) c;
                    break;
                }
                // Linear probing.
                index = (index + 1) & mask;
            }
        }
        return new SmallCharMatcher(table, filter, containsZero, description);
    }

    @Override
    public boolean matches(char c)
    {
        if (c == 0)
        {
            return containsZero;
        }
        if (!checkFilter(c))
        {
            return false;
        }
        int mask = table.length - 1;
        int startingIndex = smear(c) & mask;
        int index = startingIndex;
        do
        {
            if (table[index] == 0)
            { // Check for empty.
                return false;
            }
            else if (table[index] == c)
            { // Check for match.
                return true;
            }
            else
            { // Linear probing.
                index = (index + 1) & mask;
            }
            // Check to see if we wrapped around the whole table.
        }
        while (index != startingIndex);
        return false;
    }

    @Override
    void setBits(BitSet table)
    {
        if (containsZero)
        {
            table.set(0);
        }
        for (char c : this.table)
        {
            if (c != 0)
            {
                table.set(c);
            }
        }
    }
}
